The Truth About Metformin and Vitamin B12

Shetal Desai Rautela, RD, CDCES
March 22, 2022
4 mins

Metformin (Glucophage) belongs to a class of oral diabetes medication called biguanides — biguanides are antihyperglycemic drugs that prevent the production of glucose in the liver, improving the body’s sensitivity to insulin and reducing the amount of sugar absorbed by the intestines. Many long-term studies have shown that metformin is safe and causes very few side effects. As a result, it is a commonly prescribed medication for people with type 2 diabetes.

Metformin’s Connection to Vitamin B12

Recent studies have shown that taking metformin reduces the absorption of vitamin B12 in the intestine in up to 30 percent of patients, and that it lowers B12 levels in close to 7 percent of people with type 2 diabetes. It does takes a while — close to 3 years — for metformin to have any affect on B12 levels.

People who take metformin and follow a strictly vegetarian or vegan meal plan may be at a higher risk of developing a B12 deficiency.

Read on to learn more about B12 and how to reduce your risk of vitamin B12 deficiency when taking metformin.

The Basics of Vitamin B12

Vitamin B12 (cobalamin) is a water-soluble vitamin — which means that your body will excrete excess vitamin B12 through urine. It is naturally found in many animal-based foods.  

This vitamin is needed to form red blood cells and DNA. It has a vital function in the development of brain and nerve cells. Peripheral nerve damage — that is, nerve damage to the feet or hands — is a known complication of diabetes that is not well managed. However, another possible contributor to symptoms of neuropathy can be vitamin B12 deficiency, particularly if blood glucose levels are well-controlled.

Note: If you currently have a diagnosis of peripheral neuropathy but are unaware of your vitamin B12 levels, talk to your doctor about having your levels checked.

Symptoms of Vitamin B12 Deficiency

Talk to your care team if you notice any of these symptoms:

     • Paler than usual skin

     • Constant fatigue or weakness

     • Increased heart rate

     • “Pins and needles” sensations or numbness in your hands and legs

     • Shortness of breath

     • Blurry vision

     • Confusion

     • Mood swings or personality changes

Getting Enough Vitamin B12

Adults need 2.4 mcg of vitamin B12 per day — so where does it come from? Because vitamin B12 binds to the protein in the foods we eat, natural sources are typically protein-rich foods such as those derived from animals, including:

     • Yogurt is a great source of B12 because it is easily absorbed by the body. Choose plain Greek yogurt (1.3 mcg per 6-ounce serving) for a low-carb B12-rich option.

     • Swiss cheese (0.9 mcg per ounce).

     • 2 percent milk (1.3 mcg per 8-ounce serving) is a great source of B12, but be mindful of its effects on your blood glucose levels.

     • Organ meats (70–71 mcg per 3-ounce serving) such as liver and kidney are excellent sources of vitamin B12. But if you are watching your cholesterol levels, this option might not be the best choice.

     • Clams (17 mcg per 3-ounce serving) rank second after organ meats as a source of vitamin B12. One ounce of clams meets more than 100 percent of your daily vitamin B12 requirement.

     • Eggs (0.6 mcg per 1 large egg) are a great source of vitamin B12. The yolk has the most B12.

     • Salmon (4.9 mcg per 3-ounce serving) — a palm-size serving has more than a day’s worth of vitamin B12.

     • Tuna (2.5 mcg per 3-ounce serving) is another good source of vitamin B12. Choose the darker parts, just below the skin.

     • Chicken breast (0.3 mcg per 3-ounce serving).

     • Beef (1.5mcg per 3-ounce serving).

If you are following a vegan or vegetarian meal plan, consider talking to your care team about B12 supplements that will help you meet your recommended daily intake. There are plant-based sources of vitamin B12 available — primarily foods that have been fortified. Common forms of vitamin B12 used to fortify foods are cyanocobalamin and methylcobalamin. Look for a mention of these in the ingredients list. The amount can vary significantly, so check nutrition labels for exact amounts.
Plant-based sources of vitamin B12 include:

     • Fortified cereals (varies significantly, from around 2.7 to 24 mcg per serving) — choose high-fiber (more than three grams of fiber) cereals with less than four grams of added sugar, to support your blood glucose goals.

     • Fortified milk alternatives (around 1.8 to 2.1 mcg per cup).

     • Fortified nutritional yeast (8.3 to 24 mcg per 1/4 cup) — this is a great source of protein, too.

     • Dried shiitake mushrooms (0.8 mcg per 5 dried mushrooms).

Risk Assessment

The risk for vitamin B12 deficiency depends on the dose as well as the duration of treatment with metformin. Studies have shown that between six and thirty percent of people with type 2 diabetes who are taking more than 1,500 mg of metformin could have low B12 levels.

Knowledge Is Power — Get Tested

It’s recommended that you have your vitamin B12 levels checked every two to three years. Talk to your doctor to set up a schedule that’s right for you, as well as to learn whether a dietary supplement is needed. Low vitamin B12 levels can be treated — if you have questions about metformin and vitamin B12 deficiency, talk to your care team.

Carbon Health’s medical content is reviewed and approved by healthcare professionals before it is published, but it is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Talk to your healthcare provider about questions you may have regarding your health or a medical condition, and before making changes to your healthcare routine.

References: Association between metformin dose and B12 deficiency


Shetal Desai Rautela, RD, CDCES

Shetal Desai Rautela, RD, CDCES, is a registered dietitian and a diabetes care and education specialist at Carbon Health. She is part of our multidisciplinary home-based care and diabetes team and is passionate about personalizing meal plans that are therapeutic and enjoyable.


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